The present invention relates generally to devices and systems for controlling and regulating the conversion of power from an AC source to a load, typically a DC motor. More particularly, the present invention relates to devices which control the conduction of controllable rectifier, e.g., thyristor, bridges placed between the source and the motor and methods for monitoring the devices so as to prevent line-to-line faults caused by improper firing of the rectifiers in the bridges.
Motor control systems of the type described above typically include at least one rectifier bridge connecting the motor windings to alternating voltage supply lines. For a conventional three phase motor, each AC phase line is generally coupled to a motor by a pair of connected thyristors. That is, in a three phase system, six thyristors are required to transfer power from the source to the load, each for one half of each phase. A thyristor, such as a silicon controlled rectifier (SCR), is generally defined as a switchable diode controlled by a gate element. Each thyristor presents a relatively high blocking impedance to the flow of electrical energy until it is forward biased by a trigger current being applied to its gate element. A digital control circuit typically determines the proper time to trigger the thryistors during each half-cycle of the supply line voltage. Once a thyristor is triggered by the application of a predetermined current to its gate, the forward blocking impedance is lowered, thereby permitting the flow of electrical energy through the thyristor in the manner of a diode. Once conduction has been enabled, the thyristor cannot be turned off until the current flowing therethrough is reduced to near zero (i.e., makes a zero crossing). This occurs when the load current reaches zero. The amount of power transferred to the motor is controlled by varying the duration of the conduction of the various thyristors. This is done by controlling the firing angle of each thyristor, that is, the point during the AC waveform at which the thyristor is initiated into conduction. The process of switching from thyristor to thyristor is known as commutation.
In reversing systems or regenerative systems, that is, systems which alternatively both receive energy from the source as well as transfer energy to the source, back-to-back thyristor bridges are typically used each having a plurality of back-to-back thyristor pairs connected anode to cathode and cathode to anode. In such systems, it is necessary to accurately determine the instance at which the load current reaches zero in order to control the switching from forward bias to reverse bias modes. Absent such an accurate determination, a fault may occur effectively shorting two AC lines in the circuit when a thyristor in one bridge is conducting while any thyristor in the opposite bridge is also conducting. This kind of fault can severely damage or destroy each of the conducting thyristors. Furthermore, inaccurate measurement of the load zero current level may cause undesirable discontinuities during current reversals that could cause torque pulsations.
As described above, the turn-off point of a thyristor may be determined by measuring the load zero current level. Further, because of the possible deleterious effects of ambiguous zero current level detection, the detection of the zero current level should be made as quickly as possible. Conventionally, this measurement is made either by sensing at least two of the line currents by using AC current transformers, or by measuring the load current directly using a DC current sensor, see for example, U.S. Pat. No. 4,567,408 to Mitsuhashi. Unfortunately, many systems incorporating such power control devices utilize extremely high current/torque bandwidths, that is, currents that range from as high as 1000 amperes to as low as 0.02 amperes. Further, since conventional sensors typically generate an analog value, errors and time delays are generally introduced during the analog to digital conversion necessary to input the measurement into the digital controller. Also, due to cost concerns, the quality of conventional current sensors often limits the accuracy of the zero current level measurement. Because of the breadth of this current range, the various conversion errors and the quality of the sensor, an accurate 100% determination of the zero current level is very difficult to make. Rather, conventional controllers typically assign a zero current level bandwidth (i.e., range of values) to approximate the zero current level.
In addition to the problems and inaccuracies identified above, the amount of inductance presented at the load may also result in very long tails in the current decay, thereby increasing the time required to determine actual zeroing of the load current, and thereby delaying the time at which the reverse bridge may be initiated. All of the limitations described above combine to reduce the maximum bandwidth achievable by the system and increase the risk of shorting out the two bridges during current reversal attempts while the first bridge is still conducting.
U.S. Pat. No. 5,115,387 to Miller discloses a method for detecting thyristor conduction using additional voltage sensing means rather than conventional current sensing means. The additional voltage sensing means operate to determine whether the forward bias voltage across a thyristor exceeds a predetermined threshold voltage. If so, it is assumed that the thyristor is conducting and all thyristors in the opposite bridge are prevented from firing. Similarly, U.S. Pat. No. 3,654,541 to Kelley, Jr. et al. discloses a method for determining thyristor conduction wherein voltage detecting means operates to determine whenever the instantaneous magnitude of voltage across the thyristor exceeds a predetermined threshold level which is higher than the voltage drop across the thyristor when conducting. If so, a signal is generated indicated that the thyristor is not conducting.
Additional prior art attempts to remedy this problem include inserting additional high gain sensors into the system which are used solely for the determination of the load zero current level. Also, other attempts include measuring the voltage across the various thyristors in each bridge with a high frequency current to thereby determine precisely when the bridge stops conducting by sensing the zeroing of the added current. Unfortunately, each of the above attempts require the insertion of additional hardware into the system, thereby increasing the cost and complexity of the system.
Accordingly, there is a need in the art of power control systems for a system and method for accurately and quickly measuring the zero current level of the load so as to decrease the likelihood of cross bridge faults without increasing the cost or complexity of the system.
The present invention overcomes the problems noted above, and provides additional advantages, by providing for a method and system for determining zero current level occurrences in a reversible power converter without requiring additional component complexity and costs. A digital controller selectively determines the line to line voltage for the most recently fired thyristor pair. This line to line voltage is identified as the bridge reconstruction voltage and is compared against the actual bridge output voltage for the conducting bridge. The difference between the two voltages is identified as the bridge error voltage and the sign of its magnitude is indicative of a load current zero level occurrence. A zero current level occurrence happens whenever the bridge error voltage drops below zero. This indication is positive and instantaneous and safely enables reversing power flow without the risk of line faults due to cross-bridge short circuits.